An air conditioner that runs but does not cool is one of the most common HVAC problems, and in San Leandro it tends to show up on the first hot afternoons of late summer and early fall, after the system has sat idle for months. The cause is usually one of four things: restricted airflow (very often a clogged filter), a dirty outdoor coil that cannot get rid of heat, an electrical part that stopped the outdoor unit, or a refrigerant leak. Work through the steps below in order; each one rules something out.
Step 1: The thermostat
- Set it to Cool and a few degrees below the room temperature.
- Set the fan to Auto. With the fan on On, the blower keeps running between cooling cycles and pushes room-temperature air, which can feel like the AC is not working.
- If the display is blank or dim, replace the batteries. On a smart or scheduled thermostat, check that it is not in an away or eco mode.
- After a change, give it five minutes. Many systems wait a few minutes before restarting the outdoor unit to protect the compressor.
Step 2: The filter and vents
Pull the filter. If it is grey and matted, replace it. During long stretches of warm weather, check it monthly. Make sure the supply registers are open and the return grille is not blocked by furniture or curtains. Closing vents in many rooms reduces airflow across the indoor coil, which can make the coil freeze.
Step 3: The outdoor unit
With the thermostat calling for cooling, go outside and look and listen. Then use this table:
| What you find | What it usually means | Next step |
|---|---|---|
| Indoor fan runs, outdoor unit silent | Tripped breaker, pulled disconnect, failed contactor or capacitor | Reset a tripped breaker once; if it trips again or the unit stays silent, call |
| Outdoor unit hums, fan does not spin | Failed capacitor or fan motor | Switch off and call |
| Both units run, air only slightly cool | Dirty filter or coil, low refrigerant, or ducts leaking into the attic | Replace the filter; call if nothing improves within a few hours |
| Ice on the copper line or indoor coil | Airflow too low or refrigerant low | Thaw it (below) and call if it comes back |
| Coil fins matted with dust, leaves or lint | The system cannot shed heat on warm afternoons | Clear loose debris; schedule a coil cleaning |
| System off, water near the indoor unit | Clogged condensate drain tripped a safety switch | Call; do not keep resetting |
| One or two rooms warm, the rest fine | A duct or room problem, not the AC itself | Ask for a duct inspection |
A breaker that trips a second time points to a short in a motor, the compressor or the wiring. Leave it off.
If the coil is frozen
Set the thermostat to Off and the fan to On so air moves over the coil and melts the ice; this can take a few hours. Put towels around the indoor unit, because melting ice can overflow the drain pan. Replace the filter while you wait. Do not chip at the ice. If the coil freezes again after you restart cooling, the cause is low airflow or a refrigerant leak, and the system needs a technician.
Why “just add refrigerant” is not a fix
Refrigerant circulates in a sealed system and does not get used up. If the charge is low, there is a leak, and adding refrigerant without finding it means paying again later. Older systems that still use R-22, a refrigerant no longer produced, are especially expensive to recharge. That is often where the repair-or-replace conversation starts.
What a technician checks
- Airflow first: the filter, the blower and the temperature drop across the indoor coil.
- The electrical side: capacitor, contactor, fuses and the amp draw of the fan and compressor motors.
- The outdoor coil: its condition, and cleaning if it is restricting heat transfer.
- The refrigerant circuit: pressures and temperatures, measured only after airflow is confirmed, with a leak search if the charge is low.
- Drains and ducts: the condensate drain and float switch, plus crushed or disconnected duct runs in the attic or crawlspace.
You hear what failed and whether it is safe to keep running, and you get the price before work starts. The AC repair page covers the visit, and the AC repair cost guide explains what common repairs usually cost.
Local notes
Many homes in San Leandro, San Lorenzo and Castro Valley have ductwork in the attic. On a sunny afternoon the attic gets much hotter than the outdoors, so a leaky or disconnected duct dumps cool air where nobody feels it. The system runs nonstop and the far rooms barely cool. That calls for duct repair, not a bigger unit. Hillside homes with large west-facing windows can fall behind on the warmest days for the same reason. If your system makes new sounds along with the poor cooling, see AC noises. Heat pumps and ductless units go through similar checks; see heat pump repair and mini-split repair.
To keep this from happening, check the filter monthly in warm weather, keep the outdoor unit clear of plants and debris, and book an AC tune-up each spring, which includes a look at the coils and ducts.